The present invention relates to vibration testing systems, i.e. equipment for evaluating resistance to shock and vibration failure of test articles such as electronic or mechanical devices. The present invention is more specifically directed to an improved coupling for vibration testing.
In the vibration testing of equipment, a test system generally comprises a vibration source or shaker head, a fixture or mounting to which the test article is attached, and a connecting device or coupling which transmits the vibration or shaking from the shaker head to the fixture. Typically, one end of the coupling is bolted to the shaker head, and the other end is bolted to the fixture. Then, the test article is bolted or strapped to the fixture.
For effective transmission of the vibrational motion and forces associated with it from the shaker head to the test article, the coupling device must be quite stiff in the axial direction, i.e. in the direction between the shaker head and the fixture. The stiffness that is required depends on the vibration frequency, the latter being generally 1 Hertz to 2,000 Hertz. For higher frequencies, i.e., frequencies above 100 Hertz, the coupling generally requires a rather massive section to obtain the needed stiffness.
For higher frequency vibration testing, the displacement amplitude is generally quite small, usually on the order of a few thousandths of an inch. Displacement amplitude can be in the microinch range for extremely high frequency vibration testing. Therefore, the coupling for vibration testing must have a lash or axial play that is small with respect to the displacement involved, approaching zero lash for high frequency testing.
For many vibration testing applications, at least some angular and/or translational misalignment capability is needed for protecting the shaker head from side-loading forces. These side-loading forces can develop if there is any travel of the test fixture along an axis which is out of true parallel alignment with the natural path of motion of the constrained shaker head. Bending forces can also be introduced if the test fixture develops an angular orientation relative to the shaker head. Therefore, the coupling must permit some angular misalignment and must have an offset or translational misalignment capability where the tested article is subjected to other than strictly linear vibratory motion.
A coupling which can accommodate angular and translational offset is highly desirable for linear motion applications as well, as it can avoid the need for time-consuming, and therefore expensive, precision alignment. Precision alignment can be required in all but the shortest-stroke linear vibration, where a displacement of more than a few thousandths of an inch can damage or destroy the shaker head. Precision alignment is generally not economically feasible where the shaker head must frequently be repositioned for different test configurations.
A coupling for vibration testing ideally should have the following characteristics: (a) adequate stiffness for the frequency range of interest; (b) lash compatible with the displacement range of interest; (c) adequate misalignment tolerance; (d) compact size and light weight; (e) transportability and repositionability; (f) low initial cost; (g) low maintenance; (h) low operation cost; and (i) ease of reconfiguration for various applications.
There are three basic connector schemes currently employed for connecting a shaker head to a test fixture: rigid couplings, mechanical couplings with misalignment capability, and spherical hydrostatic bearings.
Rigid couplings have no misalignment tolerance, and are therefore not useful for testing articles that must undergo any rotation. Further, with rigid couplings, alignment is costly and time consuming.
Mechanical couplings may be designed to permit offset or misalignment capability. This type of coupling generally has a significant lash, or develops lash after only relatively short periods of operation. These therefore are not of great use for any testing except low-frequency, long-stroke vibration testing.
Spherical hydrostatic bearings are frequently employed, and are currently considered the only feasible approach for high-frequency testing where alignment capability is a significant consideration. The principal benefit of spherical hydrostatic bearing is its high axial stiffness characteristic, which results from the thinness of the oil-film separating the mating moving surfaces. These bearings also have very low friction between the spherical bearing surfaces, and have a substantially zero lash due to the preloading of a preload bolt.
Spherical hydrostatic bearings have been employed for a number of years, are well known in the art, and need not be described in detail.
These hydrostatic bearing devices do have a number of significant drawbacks. Because of the small oil-film thickness (0.5-1.0 mils) the spherical bearing surfaces have extremely narrow machining tolerances. In order to ensure that there is a film of oil at all times during the testing procedure, these bearings have complicated oil passages, oil pockets, and sealing grooves machined into them, and must be connected with oil pressure feed lines. This not only increases the expense and complexity of the devices, but also reduces their transportability and increases their weight. In addition, because the oil-film can sustain only a relatively small pressure loading, the spherical bearing surfaces must have a relatively large area. This necessitates a rather large diameter for many test applications.